The purpose of the study is to compare safety and efficacy of risperidone and plaacebo in treating children and adolescents with autstic disorder. Primary Hypothesis: 1) Risperidone will be more effective than placebo in reducing aggression, agitation, seThe purpose of the study is to compare safety and efficacy of risperidone and plaacebo in treating children and adolescents with autstic disorder. Primary Hypothesis: 1) Risperidone will be more effective than placebo in reducing aggression, agitation, seThe purpose of the study is to compare safety and efficacy of risperidone and plaacebo in treating children and adolescents with autstic disorder. Primary Hypothesis: 1) Risperidone will be more effective than placebo in reducing aggression, agitation, seThe purpose of the study is to compare safety and efficacy of risperidone and plaacebo in treating children.